1. Field of the Invention
The present invention relates to a magnetic recording medium including a lower undercoat layer and an upper magnetic layer formed in this order on a non-magnetic support body and in particular, to a magnetic recording medium having a non-magnetic powder in a lower non-magnetic layer.
2. Description of the Related Arts
As the magnetic recording medium, there are known various formats for audio and video and computer peripheral apparatuses such as a high-density floppy disc and a backup data cartridge. The magnetic recording medium is expected to have a higher density and those using metal fine particles as the magnetic powder are now becoming the main stream.
In this magnetic recording medium, as the technique for realizing a high density, various methods are considered such as increasing the coercive force of the magnetic powder and reducing the film thickness of the magnetic paint film so as to suppress the output loss. Furthermore, in order to realize a high-density recording, the magnetic recording medium should have a highly smoothed surface so as to minimize the spacing loss.
In order to simultaneously achieve the reduced thickness and flatness of the magnetic layer, for example, there is a so-called simultaneous double coating method in which a lower non-magnetic layer and an upper magnetic layer are simultaneously formed on the non-magnetic support body. This simultaneous double coating enables to prepare a magnetic recording medium having an excellent surface and a thin-film magnetic layer.
However, in this simultaneous double coating method, the surface characteristics of the upper magnetic layer and the lower non-magnetic layer respectively depend on the coating paint characteristics. For example, as is disclosed in Japanese Patent Publication 8-203060, when the non-magnetic paint and the magnetic paint have physical properties which are greatly different from each other, defects such as a paint seam, color irregularities, and thickness irregularities are easily caused.
Moreover, in the simultaneous double coating method, in order to obtain a highly smoothed upper magnetic layer, the lower non-magnetic layer should be formed so as to have an excellent surface. Accordingly, when forming the lower non-magnetic layer, it is necessary to prepare a lower layer paint in which fine particles of a non-magnetic pigment are preferably dispersed.
Furthermore, in the post processing after the paint coating (such as a calendar process, a back-coat formation process, and a slit processing), when the magnetic recording medium has a high electric resistance, electric charge may cause attachment of the magnetic recording medium and defective running, resulting in a breakage and deterioration of the shape. Moreover, when the magnetic recording medium is charged with a high electric resistance, a discharge noise is generated and dusts drawn to the magnetic recording medium deteriorate the electromagnetic conversion characteristic and the running durability. Especially a flexible disc or the like easily gets charged because of sliding with a so-called liner and shell, which generates a noise and a off-track with a high frequency.
In order to reduce the electric resistance of the paint film, a conductive material is used as the non-magnetic pigment constituting the lower non-magnetic layer and a carbon black or the like is added to the lower non-magnetic layer.
From the viewpoint of the electric resistance reduction effect in the magnetic recording medium as well as the costs, the carbon black is preferable as the non-magnetic pigment to be added to the lower non-magnetic layer. However, the carbon black absorbs a large amount of liquid and cannot achieve a uniform dispersion, greatly increasing the paint viscosity. For this, when carbon black is added to the lower layer paint, it is difficult to preferably control the surface of the lower non-magnetic layer and accordingly, it is impossible to highly smooth the upper magnetic layer.
It is therefore an object of the present invention to provide a magnetic recording medium including a lower undercoat layer which is provided under a magnetic layer and has a preferable surface and an excellent electro-conductivity, and a magnetic layer which is formed as a thin film and has an excellent electromagnetic conversion characteristic.
The magnetic recording medium according to the present invention which has achieved aforementioned object includes a non-magnetic support body coated with a lower undercoat layer formed from a non-magnetic paint prepared by kneading at least a binder and a non-magnetic powder and an upper magnetic layer formed on the lower undercoat layer, from a magnetic paint prepared by kneading at least a binder and a magnetic powder, wherein the non-magnetic powder includes a non-magnetic pigment whose surface has an area coated with graphite-shaped crystal of carbon black.
In the magnetic recording medium having the aforementioned configuration according to the present invention, the non-magnetic pigment in the lower undercoat layer is subjected to surface processing by the graphite-shaped crystal of carbon black and accordingly, the non-magnetic powder has an electric conductivity. Thus, the magnetic recording medium can reduce the electric resistance in the paint films formed on the non-magnetic support body. Furthermore, in this magnetic recording medium, the non-magnetic powder has an excellent dispersion characteristic and accordingly, the lower undercoat layer has a preferable surface characteristic.